This Invention relates to a transmission power control device for controlling a transmission power in a radio communication apparatus such as a portable telephone.
In general, a portable telephone cannot operate without a battery which is used as a power source. In order to lengthen a talk time of the portable telephone, it is necessary to reduce a current consumption as little as possible in the portable telephone. During a transmission (conversation), the current is mostly consumed in a transmission power amplifier (high frequency power IC) in the portable telephone. Therefore, it is necessary to reduce the current consumption as little as possible in the transmission power amplifier. The portable telephone may be called a mobile terminal.
It is known that the portable telephone has a conventional transmission power control device for controlling a transmission power, in order to decrease the current consumption in the portable telephone.
On the other hand, it is required to vary a transmission power over a range of 80 dB in the portable telephone in order to avoid the near-far problem caused by a distance between a base station and the portable telephone in a CDMA mobile communication system such as IS 95. When a portable telephone is far from the base station, the portable telephone is inevitably subjected to interference by another portable telephone located near the base station.
In case where the conventional transmission power control device is used in the portable telephone of the CDMA system, it is difficult to widely vary the transmission power. As a result, it is impossible to lengthen the talk time of the portable telephone as will be described later.
It is therefore an object of this invention to provide a transmission power control device capable of widely varying a transmission power in a portable telephone of CDMA system, in order to lengthen a talk time of the portable telephone.
Other objects of this invention will become clear as the description proceeds.
On describing the gist of this invention, it is possible to understand that a transmission power control device is for use in a radio communication apparatus.
According to this invention, the transmission power control device comprises (A) processing means for processing an input transmission signal into a processed transmission signal in accordance with a control signal, (B) power amplifying means having a first transistor for amplifying the processed transmission signal into a power amplified transmission signal, (C) detecting means for detecting a current flowing through the first transistor to produce a current detection signal, (D) converting means for converting the control signal into a reference value signal corresponding to a level of the processed transmission signal, (E) bias current means for comparing the current detection signal with the reference value signal to produce a comparison result signal, the bias current means controlling a bias current for the amplifying means in accordance with the comparison result signal.